OLEDs are light-emitting devices with a number of layers arranged on top of each other (layer stack) comprising at least one organic light-emitting layer (EL-layer) arranged between two electrodes (anode and cathode). The light is emitted by excited light-emitting molecules (possibly embedded in an organic matrix material) of the EL-layer. The light-emitting molecules are excited by transfer of the recombination energy of electron-hole pairs to the light-emitting molecules within a recombination zone somewhere in the EL-layer. The electrons and holes (charge carriers) are injected from the electrodes into the organic layer stack due to the operating voltage establishing an electric field in the EL layer also responsible for the transport of the charge carriers through the organic layer stack. In common OLEDs, the injection is determined by the applied voltage, the work function of the electrodes and the electrical properties of the OLED layer stack. OLEDs can only be operated in an effective way, if the OLED layer stack is well balanced. An OLED is well balanced, if the recombination zone is located within the EL-layer and the number of charger carriers of both types is suitably adjusted in order to prevent charge carriers to reach the opposite electrode (electrons→anode or holes→cathode). A recombination zone mainly outside the EL-layer would increase the losses of recombination energy via non-emitting channels. Also holes or electrons reaching the opposite electrodes are not able to excite the light-emitting molecules. Additionally, layers not designed for load with the wrong type of charge carriers could show a worse lifetime behavior. Therefore, common OLEDs further comprise additional hole and/or electron transport layers, blocking layers or injection layers of adapted thicknesses. After manufacturing the OLED, the injection of charge carriers and their transport properties through the stack can only be varied by changing the operation voltage. The charge carrier mobility is significantly different between electron and holes. Additionally, the charge carrier mobility is field or carrier concentration dependent and thus will vary with the applied voltage in a complicated manner. For instance, a recombination zone mainly located outside the EL-layer can be re-located by adjusting the operation voltage, but the charge carrier concentration and the width of the recombination zone will be also changed. This could result in a significant number of charge carriers reaching the opposite electrode or stress organic layers only designed for being loaded with the other type of charge carriers. It is therefore desirable to be able to adjust the injection of charge carriers into the organic layer stack independently from the operation voltage determining the transport of the charge carriers within the organic layer stack.